1. Field of the Invention
The present invention relates to ferroelectric liquid crystal display devices and, more particularly, to ferroelectric liquid crystal display devices capable of gradational display.
2. Related Arts
Ferroelectric liquid crystals were discovered by Meyer et al in 1975. Thereafter in 1980, applications of ferroelectric liquid crystals to such device as surface- stabilized ferroelectric liquid crystal display device were proposed by Clark and Lagerwall. This ferroelectric liquid crystal display device, in which a ferroelectric liquid crystal having a chiral smectic C phase is sandwiched between a pair of substrates with a slight clearance as small as about 2 .mu.m, utilizes a switching between two stable states provided by the ferroelectric liquid crystal. The switching is attributed to direct interaction between the spontaneous polarization of the ferroelectric liquid crystal and an electric field. Therefore, the response speed is very fast, typically, several dozens .mu.sec, which is 1000 times faster than that of a nematic liquid crystal display device utilizing a dielectric anisotropy of a nematic liquid crystal. The ferroelectric liquid crystals also have a major characteristic of a memory effect. The combination of the memory effect and fast response will realize a liquid crystal display device with a high display capacity which has more than 1000 scanning lines.
The ferroelectric liquid crystals are expected to become next-generation liquid crystals, and are now being intensively researched. One of the major problems in a practical application of the ferroelectric liquid crystals is a difficulty in realizing a tradational display. The conventional approaches to the tradational display include area division, analog gradation, and time division (or multiplexing), for which many proposals and reports have been made. However, it has never been reported or proposed that ferroelectric liquid crystal display devices having a display capacity of multi-scale gradation such as full-scale gradation or 256-scale gradation were put into commercial use. One of the conventional approaches to gradational display was reported by W.J.A.M. Hartmann in Ferroelectrics, Vol.122, pp1-26 (1991).
Among the aforesaid three approaches, the area division approach realizes a gray-scale display generated in accordance with an area ratio between bright portion and dark portion which appear on a pixel divided into plural areas by a given area ratio. To realize a 256-scale gradation, for example, one pixel is divided into partition areas by an area ratio of 2.sup.0 :2.sup.1 :2.sup.2 :2.sup.3 :2.sup.4 :2.sup.5 :2.sup.6 :2.sup.7, as shown in FIG. 12,
Though the area division approach has an advantage that, the gradation is realized by using only one kind of liquid crystal, there exist the following disadvantages:
(1) Since a greater number of electrodes and LSIs for driving are required, the production cost rises (In a case as shown in FIG. 12, the number of LSIs for driving required on the side of scanning electrodes is two times greater, and that required on the side of signal electrodes is four times greater than a liquid crystal display device having no gradation display capability);
(2) In case that the width of electrodes is narrow, the resistance of the electrodes increases, thereby reducing the sharpness of the rise of signals; and
(3) In case that matrix driving is employed for sequential operation of electrodes, fast writing speed is required (In the case shown in FIG. 12, the side of the scanning electrodes is divided into two portions and, therefore, the writing speed is required to be doubled).
Accordingly, this approach is not preferable to realize a multi-scale gradational display.
Another approach is disclosed in Japanese Unexamined Patent Publication No.3-174514 (1991) in which plural kinds of ferroelectric liquid crystals are utilized to realize a gradational display. In accordance with this approach, a plurality of partition walls are provided in a pixel between a pair of electrodes, and ferroelectric liquid crystals having different threshold voltages are filled in two or more partition areas of the pixel divided by the partition walls.
In this approach, it is essential that plural kinds of ferroelectric liquid crystals having different threshold voltages are employed depending on the number of gradational scales required for the gradational display. That is, this type of liquid crystal display devices requires an extremely complicated production process because plural kinds of liquid crystals are filled in the partition areas of the pixel, and will result in a higher production cost. Therefore, this approach is not preferable to realize a multiscale gradational display.
On the other hand, there has been proposed for the analog gradation a method of providing a gradational display by providing a gradient to the switching threshold of a ferroelectric liquid crystal filled in a pixel. However, no practical application has been realized yet.
In view of these circumstances, it has been desired to provide a ferroelectric liquid crystal display device which realizes a multi-scale gradational display with a single kind of ferroelectric liquid crystal and without increasing the number of electrodes.